Voltage transmission link for testing EMI susceptibility of a device or circuits

ABSTRACT

The performance of a device under test is observed with and without the presence of controlled EMI fields. Voltage signals from the device under test are transmitted to an oscilloscope by overdamped conductors, a hybrid electrical/optical transmitter, a fiber optic cable, a receiver and a conductor cable. The overdamped conductors are transparent to the EMI fields and the current in those conductors is kept very small by the use of a very high input impedance in the transmitter. The hybrid transmitter may be an analog or a digital transmitter.

TECHNICAL FIELD OF THE INVENTION

This invention relates to method and apparatus for testing thesusceptibility of devices, such as circuitry, to electromagneticinterference (EMI).

BACKGROUND OF INVENTION

Analog and digital electronic circuitry and attendant wiring mayencounter serious operating difficulty in the presence of strongelectromagnetic fields. Such fields are generally referred to asElectromagnetic Interference (EMI) fields. The circuits and attendantwiring may be shielded and filtered to provide some immunity to largeEMI fields, however, it is not possible or practical to design thecircuitry and attendant wiring so as to ensure immunity to EMI fields.Method and apparatus, therefore, are required to test the susceptibilityof the circuits and attendant wiring to EMI fields.

EMI testing is typically performed in shielded enclosures, known as"screen rooms", which provide an electromagnetic environment whereinonly controlled EMI fields are present. Apparatus typically used insidethe screen room includes current probes attached to a harness wire and atransmitter which sends the signals detected by the probes to a receiveroutside the screen room, where the effects of the EMI fields on thecircuit are determined.

To ensure the integrity of the screen room and the results of the EMItests, any voltage measuring apparatus within the screen room shouldminimally perturb the controlled EMI fields and should be energized by asignal from the device under test only. For example, any test apparatuswhich might reradiate EMI fields impinging on the device under test ormight otherwise inject any noise into the device under test must beavoided.

Probes suitable for monitoring current during EMI tests are commerciallyavailable. The Ailtech model number 91197-11 is one such device. Voltageprobes are available, however, none are adequate to monitor voltagesduring EMI radiated susceptibility testing. Existing wire probes act asan antenna in the screen room, picking up energy from the controlled EMIfields. The induced energy on the probes may be injected into the deviceunder test at the point of monitoring, where it can degrade performance.Existing wire probes, therefore, may cause an erroneous indication ofsusceptibility to EMI fields.

The lack of suitable wire voltage probes for use in EMI testing hasresulted in the promulgation of test procedures, in certain industries,specifying that wired instrumentation, such as voltage probes, not beused.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a voltage probe andtransmitter for monitoring the effect of EMI on a device under test.

It is a further object of the present invention to provide a voltageprobe which does not affect the device under test, or the test results,in the presence of EMI fields.

It is a further object of the present invention to provide anelectromagnetically transparent voltage probe.

It is a further object of the present invention to provide a method formeasuring the effect of EMI fields on a device under test, withouteffecting the device under test or the test results.

In accordance with this invention an electromagnetically transparentvoltage probe comprises electrically overdamped input conductors and ahybrid electrical/optical data transmitter having a high impedance inputport connected to one end of the input conductors. The other end of theinput conductors is connected to a device under test by short conductingclips. The clips' insulation is surface coated, with any bright,metallic, reflecting material such as a silver paint or foil to shieldthe clip contact from impinging EMI fields, thereby preventing theinjection of signals into the device under test by the clip. Thetransmitter has an optical output port that is connected to a receiverby way of an EMI immune optical fiber, where the effects of controlledEMI fields on the device under test are monitored.

The input conductors may comprise a thread core impregnated with fineconducting particles and an insulating sheath. The overdamped conductorshave a high distributed resistance so that they will not ring or tune atthe frequencies of interest and, therefore, will not pick up energy fromthe EMI fields. As a result, the voltage probe transmission link may beused to monitor voltages of a device under test in the presence of astrong EMI field.

THE DRAWINGS

FIG. 1 is an illustration of the voltage probe, in accordance with thepresent invention, as used in a screen room in conjunction with atransmitter and receiver.

FIG. 2 is a schematic representation of a hybrid electrical/opticalanalog transmitter in accordance with a first embodiment of the presentinvention.

FIG. 3 is a schematic representation of a hybrid electrical/opticaldifferential digital transmitter in accordance with a second embodimentof the present invention.

DETAILED DESCRIPTION

The application of a voltage transmission link in the EMI testing ofelectronic devices in a screen room 101, in accordance with theinvention, is illustrated in FIG. 1 of the drawings. The device undertest (DUT) 105 may be a component with attendant wiring resting on atest table 104 or may be a product, e.g., electronic circuit boards withattendant wiring harness and cables, which is installed in a largestructure, such as an automobile. The DUT 105, is inside an EMI screenroom 101 which provides a controlled electromagnetic environment, inwhich the antennae 102 and 103 generate EMI fields of knowncharacteristics. While the screen room provides a convenient testenvironment, it is also possible to perform EMI testing in accordancewith the invention without the benefit of a screen room.

The sources for driving the antennae 102 and 103 are not shown in thedrawing and are usually located outside the screen room 101. There arecontrols for selectively turning the sources on and off and for settingthe level of power and the frequencies of the signals applied to theantennae 102 and 103.

The transmitter 106 and the connectors 107, 108 and 109 provide thecapability to monitor points of interest in a DUT 105 having eitherdifferential or single ended outputs. If the output is a differentialsignal, the conductors 107 and 108 are connected to the signal points ofinterest and the conductor 109 is connected to ground in the DUT 105. Ifthe output is single ended, the conductor 107 is connected to that pointof interest and the conductor 109 is connected to ground in the DUT 105,while the conductor 108 is not connected.

The connection to the points of interest in the DUT 105 is made by shortconductive clips 120. Typically, the clips should have a length lessthan or equal to one centimeter to prevent the EMI electric field fromcoupling onto the clip and subsequent injection of energy into DUT 105.A one centimeter long clip will half-wavelength tune to 15 Ghz. This iseffectively outside the uppermost frequency of interest--12.4 Ghz. Toallow the convenience of operation with longer clips and to extend thefrequency range of operation to higher frequencies, the clips'insulation is coated with a conductive paint or foil, open at both ends,that effectively shields the clip from EMI. The coating also preventsthe clip from reradiating EMI that has coupled onto the DUT 105. Onesuch clip is a product number KLEPS3STO.64 marketed by Hirshmann Inc. ofAmerica. The conductors 107, 108 and 109 are connected to the inputports A, B and G, respectively, of the transmitter 106 by BNCconnectors, such as those marketed by Amphenol under product number31-1002.

The conductors 107, 108 and 109, in accordance with the invention, areof construction that renders them transparent to impingingelectromagnetic fields, so that they do not pick up any energy fromelectromagnetic fields. They may comprise a continuous monofilament coreof plastic which is impregnated with fine conductive particles, such ascarbon and covered with a plastic insulating sheath. A conductormaterial with the above described characteristics is marketed by ThePolymer Corporation of Reading, Pa. under the tradename FLUOROSINT® 719.It comprises a carbon/fluoropolymer core 0.030 inches in diameterenclosed by a transparent nylon insulating cover with an outer diameterof 0.040 inches and has resistance per length of cable within the rangeof 20,000 to 30,000 ohms per foot.

These conductors, therefore, have a uniformly distributed resistance,capacitance and inductance which causes them to be electricallyoverdamped and transparent to EMI fields. The conductors do not ring ortune at frequencies of interest in EMI testing and, therefore, will notpick up RF energy from EMI fields.

Referring to FIGS. 2 and 3 of the drawings, analog and digitalembodiments of the data transmitter 106 are illustrated. In describingboth embodiments, the A and B input ports comprise a FET inputoperational amplifier 202 and 204, such as Motorola's product numberLF357BJ. The conductors 107 and 108 are connected through a BNCconnector to the non-inverting terminal of the respective operationalamplifiers 202 and 204. Each operational amplifier is configured in thevoltage follower mode, with the output fed back to the invertingterminal, so that the input impedance is multiplied by the open loopgain of the operational amplifier. A very high input impedance is thusobtained, thereby limiting current in the conductors 107 and 108 to lessthan five nanoamperes.

The output of the operational amplifier 202 goes to the invertingterminal of differential amplifier 206 through a series resistor 208.The output of the operational amplifier 204 goes to the non-invertingterminal of the differential amplifier 206 through a series resistor210. The differential amplifier 206 provides common mode rejection fordifferential signals. The common mode signal will move in phase at thenon-inverting inputs at operational amplifiers 202 and 204. Since thesesignals have equal paths to the inputs of the differential amplifier206, they will be in phase there as well. Since the differentialamplifier 206 only amplifies the difference in signals at its inputs,the common mode signals are rejected.

The resistor 214 and adjustable resistor 216 bias the output of thedifferential amplifier 206 for linear output voltage swings across thelight emitting diode (LED) 226. Resistor 212 provides a feedback pathfor the output of the differential amplifier 206 to its inverting input.Amplifier 206 is connected for unitary gain in order to avoid overdriving the LED 226 with large (10V_(pp)) analog input signals.

Referring to FIG. 2 of the drawings, the remaining portion of the analogtransmission circuit will be described. The output of the differentialamplifier 206 is input to the base of transistor 224 through a seriesresistor 218. The resistors 218, 220 and 222 bias the transistor 224 forlinear operation. The emitter of transistor 224 is connected to theanode of the LED 226. The cathode of the LED 226 is connected to groundin series with a current limiting resistor 228. The analog opticaloutput of the LED 226 is applied to a fiber optic cable 111 by way of anoptical port 110.

The LED 226 is activated when transistor 224 is driven by the output ofthe differential amplifier 206 in response to the presence ofdifferential signals at input ports A and B. The analog transmissioncircuit of FIG. 2 is capable of transmitting non-square wave signalsfrom DC to 1 MHz. For square waves, the accurate circuit response is upto 100 kHz. A center tapped battery operated power supply is employed asshown in FIG. 2.

Referring to FIG. 3 of the drawings, the input operational amplifiers202 and 204 and the differential amplifier 206 for common mode rejectionare the same as in the analog transmitter of FIG. 2. The remainingportion of the digital transmission circuit will now be described. Theoutput of the differential amplifier 206 is connected to the invertinginput of a wideband high gain amplifier 330 through series resistor 333.The non-inverting input of amplifier 330 is grounded and its output isfed back through resistor 332 to the inverting input. Amplifier 330provides voltage gain to the differential signal.

The amplified signal is applied to a pulse restoration circuit whichcomprises a potentiometer 334, capacitor 336 and exclusive NOR gate 338.The voltage output of the amplifier 330 is a one volt signal thatdepends on the differential input signals and is offset by a three voltDC level. The potentiometer 334 is adjusted to reduce the offset to alevel where the one volt signal will cross the input threshold of gate338 to properly trigger the gate 338. The output of amplifier 206 isreduced to 1 volt at 20 MHz due to probe attenuation at frequenciesabove 1 MHz. The input to amplifier 330 is approximately 10 mV at 10MHz. A high gain amplifier is therefore required for the digital path.As a result, the gate 338 is triggered to output a fast rise time fourvolt signal when there is a differential digital signal at the inputports A and B, so that the LED 344 will output the differential signalon the optical cable 111. The diode 340 is a high speed diode thatimproves the rise time of the output signal. The gate 338 is nottriggered when there is a common mode input at ports A and B, so thatcurrent flows from resistor 342 through the forward biased diode 340,thereby abutting the LED 344 off.

The digital transmission circuit of FIG. 3 is capable of transmittingdigital signals with data rates in the range of DC to 20 MHz. Asmentioned above, the analog signal path of FIG. 1 may be used for squarewaves with frequencies less than 100 kHz. Both the analog and digitalcircuits generally transmit a linear reproduction of the originalwaveform, but this is not necessary, since EMI typically manifestsitself in much higher frequencies which tend to modulate the waveforms.It is only necessary, therefore, to measure changes in the signals undertest.

The transmission circuits of FIGS. 2 and 3 may also receive single endedsignals from a DUT 105 and transmit them to the receiver 116. This isaccomplished by connecting the B input of each transmitter to ground, sothat the non-inverting input of the operational amplifiers 204 isgrounded. A switch may be provided . .an.!. .Iadd.on .Iaddend.the frontpanel of the transmitter 106 to switch between single ended anddifferential signals.

The circuits of the transmitter 106 should be packaged in an enclosurethat shields against EMI fields as high as 300 volts per meter. Also,the transmitter is powered by an internal battery within the enclosure,thereby avoiding external power leads and the necessity of providingsufficient EMI filtering.

Referring back to FIG. 1 of the drawings, the optical port 110 of thetransmitter 106 is coupled to the receiver 116 by the fiber optic cable111. The connectors at the ports 110 and 114 are optical and the feedthrough device 112 passes the cable 111 from the screen room 101 withouttransfer of EMI fields to or from the screen room. The receiver 116 isconnected to a recorder, e.g. an oscilloscope 119, by a shieldedelectrical cable 117. The receiver 116 may be any commercially availablereceiver capable of handling the bandwidth of the output signals fromthe transmitter 106. The receiver 116 and transmitter 106 should behoused in a shielded enclosure that provides EMI immunity to fields aslarge as 300 volts per meter.

A DUT 105 may be tested for susceptibility to EMI fields in a screenroom 101, as illustrated in FIG. 1 of the drawings, as follows. Signalsfrom the DUT 105 are transmitter to the oscilloscope 119 by the hybridelectrical/optical transmitter 106, the fiber optic cable 111, thereceiver 116 and the electrical cable 117. These signals are firstobserved without energizing the antennae 102 and 103. The antennae aresubsequently energized by signals for differing power and frequenciesand the change in signal waveforms appearing at the oscilloscope areobserved. Changes in the observed signals with and without the presenceof EMI fields serve to identify areas in the DUT 105 that aresusceptible.

While illustrative preferred embodiments of the invention have beendisclosed herein, many departures from those embodiments may be madewithout departing from the spirit and scope of the claimed invention.For example, a single ended digital transmitter without common moderejection may be employed since high frequency (greater than 20 KHz)common mode noise is damped out. Further although discrete componentcircuitry has been shown, integrated circuits may be employed. Forexample, the transmitters of FIGS. 2 And 3 could be fabricated with alow cost integrated circuit.

What is claimed is:
 1. . .An electromagnetically transparent voltage transmission link.!. .Iadd.Apparatus .Iaddend.for determining the susceptibility of a device under test to EMI over certain frequencies .Iadd.above dc .Iaddend.of interest . .by monitoring voltages of said device,.!. comprising:conduction means, having first and second ends, said conduction means being .Iadd.a non-metallic material .Iaddend.electrically overdamped along its length for said frequencies of interest, .Iadd.said conduction means having a series resistance which is distributed along a portion of its length, .Iaddend.the first end of said conduction means adapted to be connected to said device under test and exposed to said EMI, and a transmission device having . .very high impedance.!. .Iadd.an .Iaddend.input port .Iadd.having an impedance that avoids loading the signal source .Iaddend.to which the second end of said conduction means is connected, for receiving at least one first signal from said device under test by way of said conduction means and for transmitting at least one second signal, related to said first signal, to a location remote from said device under test.
 2. . .An electromagnetically transparent voltage probe in accordance with.!. .Iadd.The apparatus of .Iaddend.claim 1 wherein said conduction means comprises at least two overdamped .Iadd.non-metallic .Iaddend.conductors.
 3. . .An electromagnetically transparent voltage probe in accordance with.!. .Iadd.The apparatus of .Iaddend.claim 1 wherein said conduction means comprises a thread core impregnated with fine conducting particles and an insulating layer surrounding said thread core.
 4. . .An electromagnetically transparent voltage probe in accordance with.!. .Iadd.The apparatus of .Iaddend.claim 3 wherein said thread core is a monofilament plastic . .thread.!..
 5. . .An electromagnetically transparent voltage probe in accordance with.!. .Iadd.The apparatus of .Iaddend.claim 3 wherein said fine conducting particles are composed of carbon.
 6. . .An electromagnetically transparent voltage probe in accordance with.!. .Iadd.The apparatus of .Iaddend.claim 1 wherein said conduction means has a series resistance which is distributed along its length.
 7. . .An electromagnetically transparent voltage probe in accordance with.!. .Iadd.The apparatus of .Iaddend.claim 6 wherein said conduction means exhibits a series resistance of several thousand ohms per foot thereof.
 8. . .The electromagnetically transparent voltage probe of.!. .Iadd.The apparatus of .Iaddend.claim 1 wherein said conduction means has . .a capacitance and inductance.!. .Iadd.an impedance .Iaddend.which is distributed along its length.
 9. . .An electromagnetically transparent voltage probe in accordance with.!. .Iadd.The apparatus of .Iaddend.claim 1 wherein said first end of said conduction means comprises a short conductive clip.
 10. . .An electromagnetically transparent voltage probe in accordance with.!. .Iadd.The apparatus of .Iaddend.claim 9 wherein said short conductive clip is less than or equal to one centimeter in length.
 11. . .An electromagnetically transparent probe in accordance with.!. .Iadd.The apparatus of .Iaddend.claim 10, wherein said short conductive clip is protected from impinging electric fields with electrically isolated . .silver.!. .Iadd.reflective .Iaddend.paint or metal foil on the surface of the housing.
 12. . .An electromagnetically transparent voltage probe in accordance with.!. .Iadd.The apparatus of .Iaddend.claim 1 wherein said second end of said conductor means comprises a BNC connector.
 13. The . .electromagnetically transparent voltage probe.!. .Iadd.apparatus .Iaddend.of claim 1 wherein the . .high impedance.!. .Iadd.input .Iaddend.port comprises a FET input operational amplifier.
 14. The . .electromagnetically transparent voltage probe.!. .Iadd.apparatus .Iaddend.of claim 1 wherein said . .very high.!. input . .impedance.!. port .Iadd.impedance .Iaddend.comprises the non-inverting input of a FET input operational amplifier, the output of which is reapplied to its inverting input, to provide an input impedance on the order of approximately ten . .gigohms.!. .Iadd.gigaohms .Iaddend.(10¹⁰ ohms).
 15. The . .electromagnetically transparent voltage probe.!. .Iadd.apparatus .Iaddend.of claim 1 wherein said transmission device converts an electrical signal at its input port to a related optical signal for optical transmission to the remote location.
 16. The . .electromagnetically transparent voltage probe.!. .Iadd.apparatus .Iaddend.of claim 15 wherein said conduction means has two signal leads and one ground lead, and said transmission device functions to reject common mode signals, and amplify differential signals on said signal leads, said amplified differential signals controlling the application of a driving voltage to a LED, to produce said related optical signal.
 17. A method for testing the susceptibility of a device under test to EMI fields over certain frequencies .Iadd.above dc .Iaddend.of interest . .by monitoring voltages of said device.!., comprising:attaching the first ends of at least two .Iadd.non-metallic .Iaddend.conductors to at least two points of interest in a device under test, the conductors being electrically overdamped along their length for said frequencies of interest, attaching the other ends of said conductors to respective . .high impedance.!. input ports .Iadd.having an impedance that avoids loading the signal source .Iaddend.on a transmission device, transmitting a signal related to the voltages at said points of interest to a location remote from said device under test, selectively applying controlled EMI fields to said device under test and to at least a portion of said conductors, and monitoring the effect of said EMI fields on said device under test at said remote location.
 18. The method of claim 17 wherein the step of monitoring the effect of said EMI fields on said device under test further comprises the step of comparing said signal related to the voltages at said points of interest during the selective application of controlled EMI fields to said signal during the absence of said EMI fields. .Iadd.
 19. The apparatus of claim 18 wherein the first circuit further comprises a light source and a second circuit for operating the light source to produce an optical signal that is linearly related to the input electrical signal. .Iaddend..Iadd.20. The apparatus of claim 19 wherein the light source is a light emitting diode. .Iaddend..Iadd.21. The apparatus of claim 15 wherein said transmission device further comprises a first circuit for converting the input electrical signal to a corresponding digital optical signal. .Iaddend..Iadd.22. The apparatus of claim 15 wherein said conduction means has two signal leads and said transmission device further comprises a differential amplifier, a light emitting diode and a circuit for driving the light emitting diode in response to the differential signals on the signal leads, thereby producing the related optical signals. .Iaddend..Iadd.23. The apparatus of claim 22 wherein the optical signals are linearly related to the differentially amplified signal. .Iaddend..Iadd.24. The apparatus of claim 3 wherein said monofilament core is a fluoropolymer and the finely divided particles are carbon particles and the core has a diameter of 0.030 inches. .Iaddend..Iadd.25. The apparatus of claim 1 wherein the conduction means monitors a voltage at a point of interest in the device under test. .Iaddend..Iadd.26. The apparatus of claim 15 wherein said transmission device further comprises a first circuit for converting the input electrical signal at the port to a corresponding analog optical signal. .Iaddend..Iadd.27. The method of claim 17 wherein transmitting a signal further comprises converting the electrical signal input to the transmission device to a related optical signal and transmitting the optical signal to a location remote from the device under test. .Iaddend..Iadd.28. The method of claim 27 further comprising converting the input electrical signal to a linearly related analog optical signal. .Iaddend..Iadd.29. The method of claim 27 further comprising converting the electrical signal to a digital optical signal. .Iaddend..Iadd.30. The method of claim 17 further comprising applying the controlled EMI field to the device under test and at least a portion of said conductors inside a screen room and monitoring the effect of said EMI fields outside said screen room. .Iaddend..Iadd.31. A connector for electrically connecting an electrically overdamped non-metallic conductor to a device under test to EMI comprising a conductive clip for contacting a test point of the device under test, a coating of insulation extending over the surface of the clip, and a coating of electrically conducting reflective material superimposed over the insulating layer in electrical isolation. .Iaddend..Iadd.32. The connector of claim 31 characterized in that the conductive clip is less than or equal to one centimeter in length. .Iaddend..Iadd.33. The connector of claim 31 wherein the electrically conducting material is a metallic paint or metal foil. .Iaddend..Iadd.34. A method of coupling a first circuit element to a second circuit element while minimizing the radiation introduced into the second circuit element when operating in a radiation field at selected frequencies about dc comprising:providing a non-metallic conductor that is electrically overdamped along its length for the radiation field frequencies of interest; and terminating one end of the conductor in an input circuit in the first circuit element having an impedance that avoids loading the signal source. .Iaddend..Iadd.35. The method of claim 34 wherein the conductor has a plastic thread core impregnated with fine conducting particles. .Iaddend..Iadd.36. The method of claim 34 wherein the conductor has a thread core of a fluoropolymer impregnated with fine carbon particles. .Iaddend..Iadd.37. The method of claim 34 wherein the terminating step further comprises providing an operational amplifier in a voltage follower configuration and coupling the one end of the conductor to the input of the operational amplifier. .Iaddend..Iadd.38. The method of claim 34 further comprising radiation hardening the first circuit element for the selected radiation field frequencies. .Iaddend..Iadd.39. The method of claim 34 further comprising connecting the other end of the conductor to a second circuit element. .Iaddend..Iadd.40. The method of claim 34 wherein the providing step further comprises attaching a first non-metallic conductor and a second non-metallic conductor to respective input ports in the first circuit element, the first and second conductors being electrically overdamped along their length. .Iaddend..Iadd.41. The method of claim 40 further comprising connecting the other end of the first conductor to a first point of interest of a second circuit element and connecting the other end of the second non-metallic conductor to a second point of interest of the second circuit element. .Iaddend..Iadd.42. The method of claim 40 wherein the providing step further comprises providing the first and second conductors with a plastic thread core impregnated with fine conducting particles. .Iaddend..Iadd.43. The method of claim 42 wherein the providing step further comprises providing the first and second conductors with a thread core of a fluoropolymer impregnated with fine carbon particles. .Iaddend..Iadd.44. The method of claim 40 wherein the attaching steps further comprise providing respective operational amplifiers in a voltage follower configuration and respectively coupling the one end of the first and second conductors to the respective inputs of the operational amplifiers. .Iaddend..Iadd.45. The method of claim 44 further comprising radiation hardening the first circuit element having the high impedance input ports and the respective operational amplifiers for the radiation field frequencies. .Iaddend..Iadd.46. The method of claim 40 further comprising monitoring the signals at the first and second points of interest in the second circuit element using the first circuit in response to the radiation field. .Iaddend..Iadd.47. The method of claim 46 wherein the monitoring step further comprises determining the effects of the radiation field on the second circuit. .Iaddend..Iadd.48. The method of claim 47 wherein monitoring the signals further comprises differentially amplifying in the first circuit element the signals conducted by the first and second non-metallic conductors. .Iaddend..Iadd.
 9. A method of conducting signals at frequencies above dc from a device under test operating within an EMI field to a first circuit remote from the device under test comprising:providing a conductor made of a non-metallic material electrically overdamped along its length, said non-metallic material comprising a plastic core impregnated with finely divided conductive particles and having an insulating sheath around the core; and terminating one end of the conductor at an input port of the first circuit having an input impedance that avoids loading the device under test, the other end of the conductor being adapted for receiving a signal from a point of interest on the device under test. .Iaddend. 